1. Field of the Invention
This invention relates generally to a method of treating a thin plate (hereinafter referred to as a "wafer") such as a semiconductor wafer, and more particularly to a method of forming micro holes such as contact holes in a thin film on the wafer by etching, employing as an etching mask material a resist having openings formed in desired patterns. The invention further relates to methods of removing an oxide film from within micro openings such as trenches formed in the surface of the wafer by etching and of cleaning the inside of the openings.
2. Description of the Related Art
An etching technique utilizing photolithography is considered important in the process of manufacturing a highly integrated semiconductor circuit. The following process is carried out, for example, in the formation of a contact hole for connecting an impurity region formed in the surface of the wafer and an interconnection.
On the surface of the wafer to be treated is formed, for example, a positive photoresist film which becomes soluble in alkali by reacting to light. The positive-type photoresist film is exposed in accordance with a pattern to be etched. During development of the exposed positive-type photoresist, only portions irradiated with light dissolve in an alkaline developer being used and thus an opening is formed in the photoresist film in accordance with a desired pattern. An etchant for etching an electrically insulating film on the surface of the wafer is applied over the photoresist film. This causes only the film on the wafer surface exposed by the opening of the photoresist film to be etched so as to form an opening in the wafer surface in accordance with a desired pattern.
As the level of integration of semiconductor integrated circuits has significantly increased, the circuit pattern formed on the wafer have become extremely miniaturized. Accurate etching is required for manufacturing highly integrated semiconductor circuit with high production yields.
It is extremely difficult, however, to accurately etch an extremely miniaturized circuit pattern on a wafer, as pointed out in Japanese Laid-Open Patent No. 61-148820, for example. This difficulty is due to surface tension of a treatment solution being used and to unsmooth flowing of the treatment solution on the photoresist.
Referring to FIG. 1, a common method of treating the surface of a wafer W and problems therewith are described below. The wafer W includes a silicon wafer 20, a thermal silicon oxide film 22 formed on a main surface of the silicon wafer 20, and a photoresist film 24 formed on the thermal silicon oxide film 22. Micro openings 26a and 26b having desired shapes are formed in the photoresist film 24.
The photoresist film 24 is in general a mixture of a compound having a naphthoquinone diazido structure ##STR1## and phenol resin. On the surface of the resist film 24, a methyl group (--CH.sub.3) and an ethyl group (--C.sub.2 H.sub.5) are bonded to a carbon atom C, but a polar group does not exist. Thus, the surface is hydrophobic.
Liquid supplied onto the photoresist is repelled at the surface thereof and becomes a liquid drop by its own surface tension. Therefore, etchant 28 permeates into some openings, for example, the opening 26b in FIG. 1. It may happen, however, that the liquid drop 28 repelled by the photoresist does not permeate into the other opening 26a.
Since the exposed surface of the thermal silicon oxide film 22 is hydrophilic, the thermal silicon oxide film 22 in the opening 26b is etched in the opening 26b by the liquid drop of etchant 28. In the opening 26a, however, the liquid drop of etchant 28 does not reach the surface of the thermal silicon oxide film 22, so that the film 22 is not etched there. That is, some of contact holes are properly formed by etching; however, other contact holes are not formed or are formed incompletely. The uniformed etching results make it impossible to manufacture a highly integrated circuit.
In order to solve the above problems, the above described Japanese Laid-Open Patent No. 61-148820 discloses a method in which the wafer is etched by exposure to a mixed vapor of hydrogen fluoride, water and solvent. Alcohol is proposed as one example of such in solvent.
The proposed etching process proceeds as follows.
(1) Referring to FIG. 2, a wafer W including a silicon wafer 20, a thermal silicon oxide film 22 formed on the silicon wafer 20, and a photoresist film 24 having openings 26a, 26b, formed on the thermal silicon oxide film 22 is exposed to a mixed vapor of hydrogen fluoride 30, water 32 and alcohol 34.
(2) Since the hydrogen fluoride 30, water 32 and alcohol 34 are all in their vapor phases, they are distributed uniformly over all the exposed surfaces of the photoresist film 24 and the thermal silicon oxide film 22.
(3) When the mixed vapor comes in contact with the surface of the thermal silicon oxide film 22, the hydrogen fluoride and water form hydrofluoric acid which then etches the surface of the thermal silicon oxide film 22.
(4) The alcohol 34 is cooled and thus liquified when it comes in contact with the surface of the wafer W. The liquified alcohol rinses and removes the reaction product produced by etching on the surface of the thermal silicon oxide film 22. This prevents the surface of the wafer W from being contaminated with the reaction product produced by etching.
As described above, the hydrofluoric acid is supplied in the form of vapor onto the surface of the wafer W in the proposed processing steps. The vapor reliably permeates even into the micro openings 26a, 26b and is liquified at the surface of the wafer W so as to etch the thermal silicon oxide film 22. Similarly, the alcohol vapor permeating into the openings 26a, 26b is liquified at the surface of the wafer W so as to rinse the reaction product produced by etching. Therefore, according to these processing steps, the shortcomings associated with etching using a micro pattern including uneven treatment of the openings 26a, 26b are avoided.
The above-described conventional art discloses the steps of etching a stationary wafer. In general, however, more uniform treatment can be expected by performing the etching processing while spinning the wafer. At increased spinning speeds, uniformity of the treatment results are enhanced. The etching rate is higher in a spinning wafer than in a stationary wafer.
However, application of the above-described conventional etching approach to a spinning wafer causes the following disadvantages. When the wafer with the micro openings such as contact holes, trenches, etc. formed in its surface is spinning, the air present in the micro openings and the outside air are hardly interchanged, i.e. mixed with each other. Even if the vapor of the hydrofluoric acid is supplied to the spinning wafer, the vapor does not permeate into the micro openings. As the spinning speed increases, the permeation of the vapor into openings becomes more and more difficult. Therefore, it is impossible to obtain a good result by applying the above described conventional art to a spinning wafer.
Even worse, as the dimension of the openings is decreased, the permeation of the hydrofluoric acid into the openings becomes difficult. In the case of an opening with an approximately 200 .mu.m diameter, for example, it is possible to etch the surface of the wafer within the opening on the stationary wafer as well as on the wafer spinning at 3000 rpm. However, when the diameter of the opening is reduced to approximately 3.0 .mu.m, it is only possible to etch the opening of a stationary wafer; when the wafer is spun at spinning speed exceeding 80 rpm, it is significantly difficult to obtain good etching. When the opening diameter is reduced to as far as 1.2 .mu.m or less, it is totally impossible to etch a wafer spinning at 80 rpm or more.
It is possible to etch the wafer when it spins at 80 rpm or less. But in such a case, it is still impossible to obtain uniform etching.